This invention is concerned with a video quality measure, in particular in situations where a video signal has been encoded using a compression algorithm.
When a video signal is to be transmitted from one location to another, it is known to encode or compress the signal using an encoding algorithm, such that the encoded signal can be transmitted using a lower bandwidth than would be needed without encoding. Upon reception, the encoded signal is decoded to retrieve the original signal. In many encoding techniques, a two dimensional cosines transform is performed, resulting in a series of transform coefficients, whose magnitude is quantized. So that the bandwidth can be allocated efficiently, the granularity of the quantisation, that is, the step size, is allowed to vary.
The process of encoding and decoding the video sequence can introduce distortion or otherwise reduce the quality of the signal. One way of measuring the level of distortion involves noting the opinion of viewers as to the level of perceptible distortion in a distorted video sequence, and averaging the results so as to obtain a Mean Opinion Score (MOS). However, this can be a time consuming process. As a result, it can be desirable to predict the loss of quality that a viewer will perceive in a video sequence. Although the degradation in the video quality as a result of encoding/decoding and the transmission process can be obtained by reference to the original sequence, such an approach is often inconvenient.
According to one aspect of the present invention, there is provided a method of generating a measure of quality for a video signal representative of a plurality of frames, the video signal having: an original form; an encoded form in which the video signal has been encoded using a compression algorithm utilising a variable quantiser step size such that the encoded signal has a quantiser step size parameter associable therewith; and, a decoded form in which the encoded video signal has been at least in part reconverted to the original form, the method comprising the steps of: a) generating a first quality measure which is a function of said quantiser step size parameter; b) generating a second quality measure which is a function of the spatial complexity of at least part of the frames represented by the video signal in the decoded form; and, c) combining the first and second measures.
Because the step size is derivable from the encoded video sequence, and because the complexity measure is obtained from the decoded signal, the need to refer to the original video signal is reduced. Furthermore, because in many encoding schemes the step size is transmitted as a parameter with the video sequence, use can conveniently be made of this parameter to predict video quality without having to calculate this parameter afresh. Importantly, it has been found that use of the complexity measure in combination with the step size improves the reliability of the quality measure more than would simply be expected from the reliability of the step size or the complexity alone as indicators of video quality.
According to another aspect of the present invention, there is provided a apparatus for generating a measure of quality for a video signal representative of a plurality of frames, the video signal having: an original form; an encoded form in which the video signal has been encoded using a compression algorithm utilising a variable quantiser step size such that the encoded signal has a quantiser step size parameter associable therewith; and, a decoded form in which the encoded video signal has been at least in part reconverted to the original form, the apparatus comprising: a) means for generating a first quality measure which is a function of said quantiser step size parameter; b) means for generating a second quality measure which is a function of the spatial complexity of the frames represented by the video signal in the decoded form; and c) means for combining the first and second measures so as to generate the measure of quality for the video signal.